1. Field of the Invention
The present invention relates to a zoom lens system and an image pickup apparatus including the zoom lens system.
2. Description of the Related Art
Recently, image pickup apparatuses including solid-state image pickup devices and those used in video cameras, digital still cameras, broadcasting cameras, film cameras like silver salt film cameras, etc., have become smaller with increased functionality. Accordingly, demand has increased for small, high-definition zoom lenses for use in imaging optical systems of the image pickup apparatuses.
So-called rear-focus zoom lenses, in which lens units other than a first lens unit on the object side are moved for focusing, are known as zoom lenses that meet such a demand (refer to, for example, Japanese Patent Laid-Open No. 6-34882, which corresponds to U.S. Pat. No. 5,424,869, and Japanese Patent Laid-Open No. 10-62687, which corresponds to U.S. Pat. No. 6,016,228).
In a typical rear-focus zoom lens, an effective diameter of the first lens unit is generally small compared to that in a zoom lens having a first lens unit that moves during focusing. Therefore, the size of the overall lens system can be reduced. In addition, close-range shooting (in particular, extra-close-range shooting) can be easily performed. In addition, since small, light lens units are moved, the lens units can be driven with a small driving force and focusing can be performed quickly.
In order to achieve both a reduction in the size of a camera (particularly the size in an unused state) and a high zoom ratio, collapsible (retractable) zoom lenses are commonly used. ‘Collapse (retract)’ refers to a process or a state in which distances between the lens units in the unused state are set to be smaller than those in a shooting state. Accordingly, an amount of projection of the lens system from the main body of the camera can be reduced.
In general, if the number of lenses is reduced while the refractive powers of the lens units are increased in order to reduce the size of the optical system, the thicknesses of the lenses are increased. Accordingly, when the refractive powers of the lens units are simply increased, the overall length of the optical system is typically not sufficiently reduced. In addition, it can become difficult to correct aberrations.
In addition, when a lens system is structured such that it can be collapsed in an unused state, errors like lens tilt can be unavoidably increased because of the mechanical structure of the lens system. Therefore, if sensitivity to eccentricity of the lens units is large, the optical performance can be degraded or image shake can occur during zooming.
According to a zoom lens discussed in Japanese Patent Laid-Open No. 6-34882, which corresponds to U.S. Pat. No. 5,424,869, zooming can be performed by moving only a second lens unit and a fourth lens unit. In this case, almost the entire function of changing the magnification must be provided by the second lens unit. Therefore, the refractive powers of the first lens unit and the second lens unit must be increased and sensitivity to eccentricity is increased accordingly.
In comparison, in a zoom lens discussed in Japanese Patent Laid-Open No. 10-62687, which corresponds to U.S. Pat. No. 6,016,228, sensitivity to eccentricity of a first lens unit and a second lens unit are relatively small. Therefore, this zoom lens facilitates a retractable structure.